Introduction

• Metamorphic rocks are formed from pre-existing rocks through heat, pressure, and chemically active fluids.
• Classification is based on texture and mineral composition.
• Two main categories: Foliated and Non-foliated rocks.
• Note: Metamorphic rocks generally do not have specific names; they are usually descriptive.

Foliated Metamorphic Rocks

1. Slate
2. Phyllite
3. Schist
4. Gneiss

• Foliation: Planar arrangement of mineral grains, giving the rock a layered or banded appearance.

Slate

• Texture: Very fine-grained, slaty cleavage.
• Common minerals: Quartz, mica, chlorite.
• Description: Derived from shale; low-grade metamorphism.

Source: https://openpress.usask.ca

Phyllite

• Texture: Fine-grained, wavy surfaces with a sheen.
• Common minerals: Mica, quartz, chlorite.
• Description: Higher grade than slate; has a silky sheen due to fine mica flakes.

Source: https://openpress.usask.ca

Schist

• Texture: Medium to coarse-grained, schistosity (visible foliation).
• Common minerals: Mica, quartz, feldspar, garnet.
• Description: Higher grade than phyllite; often has large mica grains.

Source: https://openpress.usask.ca

Gneiss

• Texture: Coarse-grained, banding of light and dark minerals (gneissic texture).
• Common minerals: Quartz, feldspar, biotite, amphibole.
• Description: High-grade metamorphic rock; distinct mineral banding.

Source: https://openpress.usask.ca

Non-foliated Metamorphic Rocks

• Non-foliated texture: No preferred orientation of mineral grains.

Marble

• Texture: Coarse-grained, granular.
• Common minerals: Calcite or dolomite.
• Description: Derived from limestone; reacts with acid.

Source: https://openpress.usask.ca

Quartzite

• Texture: Medium to coarse-grained, interlocking quartz grains.
• Common minerals: Quartz.
• Description: Derived from sandstone; very hard and resistant to weathering.

Source: https://openpress.usask.ca

Hornfels

• Texture: Fine-grained, dense, and hard.
• Common minerals: Variable, often with biotite, feldspar, and pyroxene.
• Description: Formed by contact metamorphism; lacks foliation.

Source: https://openpress.usask.ca

Metamorphic Rock Names

• Descriptive Naming: Metamorphic rocks do not have fixed names.
• Rock names generally describe the texture and mineral composition.
• Example: Biotite schist, Staurolite-Garnet-Mica Schist … etc

Classification based on protolith composition (original rock)

• Each category in this classifications represents a specific chemical and mineralogical composition of the protolith

• Six primary classes:

  • Pelites
  • Psammites
  • Calcareous rocks
  • Quartzo-Feldspathic rocks
  • Mafic rocks
  • Ultramafic rocks

Pelites

• Protolith: Mudstone, shale (clay-rich)
• Composition: High in aluminum
• Common Metamorphic Minerals: Mica, garnet, kyanite, sillimanite
• Examples:
  • Before Metamorphism: Shale, mudstone
  • After Metamorphism: Slate, phyllite, schist, gneiss

Psammites

• Protolith: Sandstone (quartz-rich)
• Composition: High in silica (quartz, feldspar)
• Common Metamorphic Rocks: Quartzite, meta-sandstone
• Examples:
  • Before Metamorphism: Sandstone
  • After Metamorphism: Quartzite, meta-sandstone

Calcareous Rocks

• Protolith: Limestone, dolostone (carbonate-rich)
• Composition: High in calcium carbonate
• Common Metamorphic Minerals: Calcite, dolomite, diopside, wollastonite
• Examples:
  • Before Metamorphism: Limestone, dolostone
  • After Metamorphism: Marble, calc-silicate rock

Quartzo-Feldspathic Rocks

• Protolith: Granitic rocks, arkosic sandstones
• Composition: High in quartz and feldspar
• Common Metamorphic Rocks: Gneiss, granulite
• Examples:
  • Before Metamorphism: Granite, arkosic sandstone
  • After Metamorphism: Gneiss, granulite

Mafic Rocks

• Protolith: Basalt, gabbro (iron and magnesium-rich)
• Composition: Moderate silica, high iron and magnesium
• Common Metamorphic Minerals: Amphibole, pyroxene, plagioclase
• Examples:
  • Before Metamorphism: Basalt, gabbro
  • After Metamorphism: Amphibolite, granulite

Ultramafic Rocks

• Protolith: Peridotite (low silica, high magnesium and iron)
• Composition: Very low silica, high Mg and Fe
• Common Metamorphic Minerals: Olivine, serpentine, talc
• Examples:
  • Before Metamorphism: Peridotite
  • After Metamorphism: Serpentinite, talc-schist

Summary Table

• Metamorphic rock classification reflects original rock composition.
• Protoliths influence mineral assemblages, textures, and final rock type.
• Recognizing these classes aids in interpreting metamorphic processes and conditions.

Another Genetic Classification

Para- Metamorphic Rocks

• Prefix “Para-”: Indicates a sedimentary protolith.
• Examples:
  • Para-schist: Derived from shale or mudstone.
  • Para-gneiss: Originates from sandstone or siltstone.
• Characteristics: May retain sedimentary features, such as layering.

Ortho- Metamorphic Rocks

• Prefix “Ortho-”: Indicates an igneous protolith.
• Examples:
  • Ortho-gneiss: Derived from granite or granodiorite.
  • Ortho-amphibolite: Originates from basalt or gabbro.
• Characteristics: May retain igneous minerals and textures.